Draw floorpan to extrude?
Just start from a single vertex (make a plane and delete 3 out of 4 vertices) and extrude it like so. You can make sure your vertices are aligned by turning snapping to grid ON, or selecting multiple vertices that need to be aligned and scaling them to 0 on the relevant axis.

Once you finish your shape, select all vertices and press F to create a face, then use the Solidify modifier or extrude the face.

Does that answer your question?